The world's known oil reserves are dwindling at an ever increasing rate as developing nations industrialize and demand increases. The price of oil exceeded $100 per barrel in 2008 and is very likely to become even more expensive in the future. For electricity generation, there are many alternatives to oil-fired power stations: natural gas, coal, nuclear and hydro-electric power stations are already widely deployed throughout the United States and other industrialized nations. However, burning both natural gas and coal leads to an increase of carbon dioxide levels in our atmosphere and as global warming accelerates and governments seek to address this growing concern, there has been much recent interest in renewable energy sources such as solar, wind and tides. It should be mentioned that although the percentage of our electricity generated by nuclear energy might increase in the future, this is no panacea. The public remember incidents at Chernobyl and Three Mile Island, and there are serious concerns about the radioactive waste that will remain hazardous for hundreds if not thousands of years. Furthermore, the use of nuclear energy for peaceful purposes nevertheless boosts the supply of fissionable fuel and increases the likelihood of nuclear proliferation with all its concomitant problems.
A multi-pronged problem such as this requires a strategy that incorporates several solutions. The aforementioned increase in the adoption of renewable sources of energy is a good start, but the world must also learn to reduce its energy consumption per capita and use its energy sources more effectively. One critical component needed to achieve these goals is efficient energy storage. Here again there will be many solutions: pumping water uphill, storing compressed gas in underground caverns, converting excess electrical energy to fuels such as hydrogen, flywheels, batteries and capacitors, just to name a few. Each solution has its preferred applications and currently, batteries and capacitors are the preferred methods of storing electrical energy in small and medium-sized portable electrical appliances. However, there is growing interest in the use of larger batteries and capacitors for vehicular propulsion and load leveling or power conditioning applications. Batteries and capacitors have also been proposed for storing energy from wind and photovoltaic generators to provide power at times when the wind is not blowing or it is dark. Finally, a new class of thin-film batteries is emerging for use with MEMS (Micro-ElectroMechanical Systems), SiP (System in a Package) and other microelectronic devices.
As with most industrial operations, it requires energy to manufacture batteries and capacitors. Moreover, these devices do not, per se, create energy but they can result in more efficient use of energy. Therefore, it is important to consider the net energy balance of a particular battery or capacitor in a given application. If the energy storage device ends up saving more energy over its lifetime than was used in its fabrication, it results in valuable energy savings and likely reduction in overall CO2 emissions. If however the reverse is true, the impression that the technology in question is a “green” energy-conserving technology is illusory. Rechargeable battery manufacturing is a relatively energy intensive operation: high energy density lithium-ion batteries in particular require high purity materials, some of which must be prepared at high temperatures. Many early lithium-ion batteries had limited cycle lives of just a few hundred cycles and their net energy balance in many typical portable electronic applications was negative. They did provide better performance for a given size and weight and therefore reduced the overall size and weight of the device—before the severity of global warming and diminishing energy reserves was fully appreciated, this was the primary consideration. For vehicular propulsion and power station applications, it is critical that the net energy balance of the batteries is positive and that their lifetimes are sufficient to justify their use. By their very nature, the electrodes in electrochemical batteries undergo chemical changes during charging and discharging. These can be in the form of phase changes, structural changes and/or volume changes, all of which can severely degrade the integrity of the electrodes over time and reduce the capacity of the battery. Indeed, the charging and discharging processes in the latest generation lithium-ion batteries must be carefully controlled—overcharging or over-discharging can limit the performance and cause premature failure of the battery.
In contrast, capacitors store their energy as electrical charge on the electrodes. No chemical changes are involved and most capacitors have cycle lives of a million cycles or more, to 100% depth-of-discharge. Capacitors can also be charged and discharged orders of magnitude faster than electrochemical batteries making them particularly attractive for capturing rapidly released energy such as in falling elevator and automobile regenerative braking applications. Traditional electrostatic and electrolytic capacitors are used widely in electrical circuit applications but can store only relatively small amounts of energy per unit weight or volume. The emergence of electrochemical double layer (EDL) capacitors has now provided a viable alternative to traditional electrochemical batteries where power density and cycle life are more important than energy density. In fact, the latest generation EDL Supercapacitors have specific energies of ˜25 Wh/kg, approximately the same as lead-acid electrochemical cells.